I both sorta like the Chrysler new transmission numbering system on one hand and on another, it can be confusing.
The (automatic) transmissions that originally came in FMJ's are:
A727, A904, A998 and A999.
Of the last three, the A904 is light duty - and phased out/last used behind a 318 in '78. They still used it on /6 for later years - but for this post, I will not be mentioning the /6 - only small blocks.
A998 Medium duty
A999 Heavy duty
The A727 is a physically larger (and heaver) transmission than the other three are (very few interchangeable parts between A727 and the A904/A998/A999. The last three share a majority of parts with each other. For the most part, the A727 was only available on special order FMJ vehicles.
The A998 was later phased out in '80, leaving the A999 the only choice afterwards.
Some people might even debate if the A999 wasn't a better choice than the A727 was - but I will leave that debate up for another thread.
The A904/A998/A999 is physically smaller and lighter (than A727). Starting in mid-year '80, also have the better first gear ratio (which is much improved over the pre-mid-year '80 first gear ratio). The better first gear ratio is something that never made its way to the A727.
A727 to Left and A904/A998/A999 to Right. These are 4*4 transmissions and orange arrow shows the differences there.
Basically, the output shaft and extension housing is different. for the overdrives, basically the entire rear half is different.
Chrysler still used the A727 and A999 on pickups/vans/Dakotas for a long time after FMJ production stopped - and renamed the A727 to 36RH and A999 to 32RH.
The first digit is number of forward gears (3 - three speed)
Second digit is the amount of torque it can handle (0=weak to 9=very strong)
Third digit is R=rear drive and T=transverse (front wheel drive).
Last digit is H=hydraulic or E=electronic
In '88 (for pickups/vans/Dakotas) and A999/31RH was used to make the A500 4-speed overdrive transmission. For the sake of discussion, the front half of the A999 is the (mostly) the same but an overdrive was added onto the rear (or back-half of transmission).
They were renamed to 40RH, 42RH and 44RH (weak to strong), then in '93 (I think) it went electronic, or 40RE, 42RE and 44RE.
The "-RE's" need a computer to operate it - so best to find an "-RH" version.
The easy way I know to tell the difference between RH and RE is to look for the speedometer gear adapter or lack there of, for the RE's use a speed sensor (instead of speedometer gear) mounted on the rear differential. There were a few RE's that still used the speedometer gear hole, but for the most part, the speedometer gear adapter hole is the best way to tell them apart.
This the A518/46RH variant.
In '90, the A727 did the same and it was called the A518 - or later called 46RH/46RE. Diesel and V-10 used a heaver duty version called 47Rh/47RE but also have a different engine bolt pattern from small/big blocks. Much later on, the diesel went to 48RE.
Unless you have a monster big block (or similar), there is no need to find the bigger version - so an 40RH, 42RH or 44RH is all you need. I think the 44RH is only a 4x4 transmission so makes no sense to find one (too many differences). A V-6 with 4speed will be an 40RH. It will work, but I think I would look for an 42RH if you can.
Mostly, the big differences from 40- and 42- (or 44-) is there are drums have allowances for more clutch disks. The more clutch plates, the more torque it can handle. Even with the A904, A998 and A999 - You can (typically, but not always) insert the higher capacity A999 clutch drums unto the A904 (or A998) to increase the torque capacity.
Then you have the torque converter differences.
Across the board, until mid-year '78 all Chrysler transmissions were non-lockup. Non-lockup transmissions were available well into the mid/late '80's by special order (not common, but out there).
The '68-78 A904 input shaft spline count on non-lockup is 27.
The '66-78 A727 input shaft spline count is 24.
The '78-1/2 and newer A904/A998/A999 input shaft spline count is 26.
The '78-1/2 and newer A727 input shaft spline count is 23.
There is no reason to count the splines, though. Just look at the tip of the input shaft:
The lockup version will always have the non-splined end.
Next difference is the torque converter hub. All FMJ Chrysler automatics had slots machined into them, where they interact with the front pump gears. The slots have a tendency to crack, like this one:
And that will wipe out the front pump bushing in no time (is not a good thing).
Sometime in the '90's Chrysler went to machined flats for the front pump and that design is far superior:
A better picture of the differences:
The front pump gears can be swapped out fairly easily, as a set to accommodate the torque converter hub differences (either direction):
A904/30RH/A500/42RH on Left and A727/A518/46RH on the Left.
If I was building a Chrysler automatic transmission (and I will be, a couple - soon), I will want the flats on hub as opposed to the slots.
When a torque converter company rebuilds (or makes new) a converter, they automatically cut off the converter hub and will re-weld a new hub in its place - so easy to ask for a slot vs. flat hub.
When the lockup converter first came out, some people had problems with theirs. Very little of that design has changed over the years - but, for the most part, it is a pretty good system. Racers prefer non-lockup, and those who drive all of the time, prefer lockup (which gives about ~10% better fuel mileage). I think some racers might be missing out by not racing with a lockup - but, again, this might be a better topic for another thread.
The "adapted" A518 going behind my big block will have lockup (with a flat hub converter).
If you happen to have a good torque converter (either style) but it has the wrong front pump gears in transmission - go purchase a gear set (picture above), swap 'em out, and get-er-done.
If you have to replace a converter (which in your case, I can pretty much bet big bucks that yours is full of metal) and are going through the transmission, then getting flat hub converter/gears is the only way to go.
When the Magnum engines came out, the flexplate and torque converter attachment changed vs. the style used on FMJ's and are not really interchangeable - BUT as a pair (flexplate & converter), you can can easily put a non-Magnum converter/flexplate into a newer vehicle or use Magnum Fuel Injection on an older 318/360 by installing the magnum converter/flexplate on a FMJ transmission (once you adapt the crankshaft position sensor slot onto the bell housing).
A lot of information to digest and hopefully I made the water less muddy.
BudW